Parametric devices are used for coherent light amplification, for the generation of coherent light at frequencies where no lasers are available (e.g. in the ultraviolet band) and for detection of weak light at wavelengths for which sensitive detectors do not exist. A parametric amplifier is a parametric device consisting of an optical nonlinear crystal in which power supplied by a laser in one optical frequency is translated into other parametric frequencies. A parametric amplifier can be easily modified into a parametric oscillator, which is a device that produces a coherent beam of light tunable over a wide range of wavelengths. The prior art has developed new types of crystals that may be used in optical parametric oscillators for various spectral regions. In particular, nonlinear optical crystals capable of producing parametric output in the mid-infrared region (2-5 um), which have been developed for commercial applications, include potassium titanyl phosphate (KTP), potassium titanyl arsenate (KTA), lithium niobate (LiNbO.sub.3), potassium niobate (KNbO.sub.3), silver gallium selenide (AgGaSe.sub.2), and silver gallium sulfide (AgGaS.sub.2).
It has been determined that AgGaSe.sub.2 has an irregular or anomalous absorption band near 2 micrometers for light that propagates as an extraordinary wave in the crystal. An extraordinary wave may be defined as a wave whose polarization vector lies in a plane that contains the propagation direction and the symmetry axis of the crystal. One of the prior art approaches for mid-infrared generation is based upon applying a two micrometer laser to an AgGaSe.sub.2 optical parametric oscillator. This results in significant absorption of the (extraordinary) pump radiation which limits the prospects for average power scaling. Indeed, severe crystal heating and thermal lensing occurs at average power levels which exceed one watt. Conversion efficiency saturates even below these power levels, thereby precluding the usefulness of AgGaSe.sub.2 for many high power applications. Also, when a fixed laser is used to generate tunable waves from a AgGaSe.sub.2 crystal an electric field has to be applied to the AgGaSe.sub.2 crystal, or the AgGaSe.sub.2 crystal has to be temperature tuned.
Reference may be had to the following patents for further information concerning the state of the prior art (all of the references are incorporated herein by reference).
U.S. Pat. No. 5,159,487, issued Oct. 27, 1992, entitled "Optical Parametric Oscillator OPO Having A Variable Line Narrowed Output" to Geiger et al. describes an optical parametric oscillator (OPO) that includes a pump laser for producing a pump beam; an optical resonator; an OPO crystal disposed within the optical resonator aligned with and responsive to the pump beam to produce a parametrically generated output; and a device external to the optical resonator for line narrowing the parametrically generated output.
U.S. Pat. No. 5,144,630, issued Sep. 1, 1992, entitled "Multiwavelength Solid State Laser Using Frequency Conversion Technique" to Lin, describes an apparatus for producing multiwavelength coherent radiations ranging from deep ultraviolet to mid-infrared. The basic laser is a pulsed Nd:YAG or Nd:YLF laser which is frequency converted by a set of novel nonlinear crystals including D-CDA, LBO, BBO, KTP and KNBO.sub.3 where efficient schemes using noncritical phase matching and cylindrical focussing are employed.
U.S. Pat. No. 5,117,126, issued May 26, 1992, entitled "Stacked Optical Parametric Oscillator" to Geiger, describes a stacked OPO wherein two optical parametric crystals are coaxially disposed in a single resonator. Incident radiation is coupled to the resonator and causes parametric oscillations of the two crystals. The two crystals are independently tuned, such as by angular orientation to produce distinct components of secondary radiation.
U.S. Pat. No. 5,079,445, issued Jan. 7, 1992, entitled "High Output Coupling Cavity Design For Optical Parametric Oscillators" to Guyer, discloses a cavity design for use with a nonlinear medium which may be used as an oscillator using pump energy with frequency (FP) interacting with the nonlinear medium for parametrically generating outputs having a signal frequency (FS) and an idler frequency (FI). The parametric radiation which is produced satisfy the relationship which is common for optical parametric amplifiers and oscillators FP=FS+FI.
U.S. Pat. No. 5,070,260, issued Dec. 3, 1991, entitled "Ultrahigh-Resolution Optical Parametric Oscillator Frequency Measurement and Synthesis System" to Wong, discloses one or more parametric oscillators which are arranged selectively, singly, serially, and/or in parallel and each oscillator is responsive to an input pump beam having a fractional stability to produce output signals and idler beams having fractional stabilities that correspond to or are better than the fractional stability of the pump beam and in such a way that the sum of the frequencies of the output signal and idler beams of each optical parametric oscillator is constrained to be equal to the frequency of the input beam thereof.
U.S. Pat. No. 5,047,668, issued Sep. 10, 1991, entitled "Optical Walkoff Compensation In Critically Phase-Matched Three-Wave Frequency Conversion Systems" to Bosenberg, discloses a walkoff-compensated frequency conversion system such as an optical parametric oscillator including a pair of nonlinear crystals such as: Beta-Barium Metaborate, aligned in an optical cavity with their optical axis at an angle with respect to the axis of the cavity.
In U.S. Pat. No. 4,884,277, issued Nov. 28, 1989 to Anthon et al. there is disclosed an intra-cavity frequency-modified laser of improved amplitude stability which is obtained through the use of a plurality of nonlinear optical crystals within the laser cavity.
It is evident that it would be desirable to overcome the disadvantages of the prior art by providing an apparatus that utilizes a AgGaSe.sub.2 crystal and a mid-infrared conversion approach without causing crystal heating and thermal lensing at average power levels exceeding one watt. Such a device should also be able to generate tunable waves from a AgGaSe.sub.2 crystal by using temperature or angle tuning.